Car Forum World

i have a '93 Pathfinder that ran with the thermostat stuck open for 40,000 miles., I replaced it when I got the car from my dad, but it missed lots and ran rough when engine was cold. now, it also misses some and runs rough when warm, too. but here is the strange part.if I turn the A/C on, the engine smoothes out.why is this?? you'd think i'd be the reverse, would not you? I tried replacing spark plugs and wires, did not help at all. my hypothesis was that there were carbon deposits on the inside of the combustion chamber caused by running cold for so long, but why does it smooth out when the a/c is on?

The Idle air control AC idle control speeds up the engine masking the roughness. attempt squeezing the fuel return hose yes one of the two hoses on the passengers side of the running motor squeeze the supply hose engine dies attempt the other. If the engine smooths out look for fuel system related problems clogged injectors. And did you take off the distributor plug look into the connectors for any bent pins or pushed back connections. Some things to attempt replace the fuel filter above the rear axle? have you driven this truck like dad never did?

Nissan Master Tech

Sounds like it may be a vacuum leak.

Check all the little hoses and fittings.

Good luck.

Better go back to the 1st thing you have done in the car for the answer you seek. Your solution could just be there, waiting for you on your 1st work on the car, the substitute.

leave out the thermostat permanently. attempt it unless you live in a cold state. good luck.

I'm not exactly sure why it could do that, but, when the air conditioning is on it dumps more heat on the engine warming it up which explains that. If you have not changed the oil recently that may help too.

really the only possible answer may be,,is when the air is on,its running in a strain,but like you said,,why now,,there is one other possibility, it may have a bad temperature sensor for the computer,,since it was run cold for so long,it may not work now,,when it gets hot,,i once worked on a Toyota corolla that ran good with the air on,,but missed when it was off,,it had a bad temp sensor on it, ,i own a fix shop,,and have run across some strange things in the past 35 yrs of doing this,,you may attempt a temp sensor,,it may fix it,,good luck with it,,i hope this help,s.

Have you checked on the electric clutch on the AC unit.